As digital storage becomes more and more ubiquitous, users store media data increasingly on mobile and fixed devices that are connectable to or a part of a media replaying device. Especially, in the case of mobile devices like MP3-players or mobile video players, users may connect their devices to a home multimedia center, a home video system, a home stereo system or a car stereo system to replay media data stored on the device. Wireless connections are available as an alternative or in addition to cable connections.
Many kinds of devices are available that can store music or other audio files or that provide audio streaming. In addition to playback devices like MP3-players or CD-players, there are mobile devices that contain an audio player application in addition to a number of further applications like a mobile phone, a camera etc. For example, a number of mobile phone models contain an audio player (for example an MP3 player) or a video player. Further, also mobile computers like laptops are used to store and replay music or audio collections.
Audio can be stored in a large variety of digital and analogue formats. For example, the audio may be stored in a memory in a digitally coded form like MP3, Ogg Vorbis, AAC (advanced audio coding) or PCM (pulse code modulation), or it may be received through a wireless or wired connection such as a mobile or fixed line internet connection in a digital form.
Mobile devices often do not have high quality loudspeakers due to their size constraints. Their power typically comes from small batteries and they cannot support high quality loudspeakers for a long time. The preferred listening method for a mobile apparatus may be by headphone. However, a user may want to listen to his audio or music collection at home with the home stereo system or in a car through the car stereo system. Different connectors may be needed for cable connections, and therefore more than one cable may be needed for connecting the mobile device to more than one other device. A cable connection may break under mechanical stress. Instead, a wireless connection may be used. For example, stereo audio may be transmitted from a mobile device through a Bluetooth™ connection using A2DP (advanced audio distribution profile). A2DP is defined in the Bluetooth™ specification.
Apart from Bluetooth™, several short range radio systems may provide a wireless connection for media transfer. Point-to-point transmission systems (like WLAN or WiMAX) and broadcasting and/or multicasting may be used. In the area of broadcasting, there are analogue and digital transmission systems. FM (frequency modulation) transmission is an analogue transmission using licensed radio frequencies. FM receivers can be found in many homes, in many car stereo systems and also in a number of mobile devices. In addition, unlicensed low power FM transmission is now allowed in many countries including the United States of America, where this is regulated in FCC rule 15.
In unlicensed FM transmissions, the audio player “mimics” a conventional FM radio station and transmits the audio data encoded as a standard FM radio broadcast transmission. Additional features that are defined for FM transmissions, like RDS (radio data system), can be used in unlicensed FM transmissions as well. Unlicensed FM transmitters can be used in the car or home environment in order to replay the stored audio content of the audio player on the car radio or home stereo system.
Examples of digital broadcast transmissions systems are DAB (digital audio broadcast), DMB (digital media broadcast), DVB (digital video broadcast), and MediaFLO™ (media forward link only).
An unlicensed transmitter (either analogue or digital) can “mimic” a conventional licensed transmitter, so that the digital transmission can be received by a receiver, for example a receiver designed to receive transmissions from a licensed transmitter of the system.
Devices may be equipped with more than one transmitter/receiver in order to provide communication capability for different requirements, like different data rates, but also to provide for increased availability. A mobile device may comprise a Bluetooth™ interface, an FM transmitter/receiver, an infrared interface, a WLAN interface and a DVB-H transmitter/receiver in addition to a cellular interface like GSM (global system for mobile communications).